Laser Weld Pool Research Articles

Feature Extraction of Laser Welding Pool Image and Application in Welding Quality Identification

Most of the existing laser welding quality identification methods are post-weld identification or low-speed identification (Welding speed below 120m/min). Efficiently online monitoring of laser welding can take the advantages of laser welding for high-speed and deep-penetration welding. How to eliminate interference information (such as metal vapor, plasma splash, etc.) in the laser welding process, accurately and quickly extract the feature information of welding quality evaluation, and identify defects is a major problem that laser welding online monitoring technology needs to solve urgently. In this paper, the optimized dark channel prior anti-interference processing algorithm can remove the interference of image. The feature information extraction algorithm based on contour and OTSU threshold segmentation are used to extract the features of the welding image that collected by the image acquisition system. Then, the image is classified as a specific defect by the trained BP neural network classification algorithm. Experiments with 304 stainless steels have proved that this method can effectively remove the interference of metal vapor and plasma splash on the feature information, and achieves 97.18% accuracy rate of the binary classification test and 91.29% accuracy rate of the six-classification test. The processing time of the entire algorithm is about 0.3ms and it can meet the real-time requirements of high-speed laser welding.

Multi-physics multi-scale simulation of the solidification process in the molten pool during laser welding of aluminum alloys

The solidification microstructures directly determine the performance of laser welds. However, the present understanding of the dynamic solidification behavior within the melt pool during laser welding is still limited. This paper developed a multi-physics multi-scale model to investigate the complex solidification process during laser welding of 5083 aluminum alloy. The model combines the macroscale model for heat and mass transfer, the microscale model for polycrystalline alloy solidification, and the continuous Gaussian nucleation distribution model for heterogeneous nucleation. The model is validated by comparing with experimental results from three aspects, namely, the fusion profile, the grain structure and the dendritic structure, and good agreements are achieved. The whole solidification process, from the planar growth to cellular growth, columnar dendritic growth and equiaxed dendritic growth, in the laser weld pool is studied through analyzing the evolution of the temperature, solute concentration and constitutional undercooling fields. In particular, the distribution of constitutional undercooling in the liquid ahead of the solidification front is predicted quantitatively. The relationship between the constitutional undercooling distribution and the nucleation/growth of equiaxed grains is demonstrated, which reveals the selective growth behavior of the columnar grains and equiaxed grains at different locations in the laser weld.

Multi-scale simulation of grain/sub-grain structure evolution during solidification in laser welding of aluminum alloys

This work developed a multi-scale model, combining the macro-scale model for heat transfer and fluid flow with the micro-scale phase-field model for polycrystalline alloy solidification, to investigate the grain/sub-grain structure evolution during solidification in laser welding of 5083 Al sheets. Special attentions were paid to the planar to cellular and dendritic growth. Results demonstrated that, during planar-to-cellular transition, the initial planar instability primarily resulted from the rapid change in growth rate. The surface tension anisotropy can significantly affect the initial planar instability. With the increase of misorientation angle, the interfacial stiffness tended to increase, which enhanced the stabilizing effects of surface tension and thus made the S/L interface more and more stable. Furthermore, Results elucidated how solidification morphology develops during cellular-to-dendritic transition. Two important phenomena, namely, tertiary branching and tip-splitting, which were responsible for the decrease in primary dendritic arm spacing, were discovered. Finally, results illustrated how multiple grains with various orientations grow in competition in laser weld pool. Two competitive mechanisms of grain growth were revealed, namely, lateral expansion of favorably-oriented (FO) grain and blocking its unfavorably-oriented (UO) grain neighbors, and the elimination of UO grain by FO grain at the converging grain boundary.

Thermoelectric currents and thermoelectric-magnetic effects in full-penetration laser beam welding of aluminum alloy with magnetic field support

Thermoelectric currents (TECs) caused by Seebeck effect were usually neglected in welding simulation. This paper characterizes the TECs and thermoelectric-magnetic phenomena during the magnetically supported laser beam welding (MSLBW) of thick aluminum (Al) alloy. The simulation is based on a computational fluid dynamic (CFD) model developed in three dimensions. The model considers multi-physical mechanisms including thermal transfer, solid–liquid (S/L) transition, molten metal convection, magnetohydrodynamics (MHD) and Seebeck effect. The computed TEC distribution in laser welding pool highly depends on welding time and temperature gradient. The large current densities occur near the weld pool edge and the vectors are opposite at both side of S/L interface. The maximum TEC density obtained in the stabilized weld pool is 2.14 × 106 A/m2, leading to a self-induced magnetic field of 2.27 × 10−3 T and a Lorentz force of 2.62 × 103 N/m3. The influences of TECs on weld pool dynamics are quite limited in LBW but notable in MSLBW. The inversion of MF direction leads to different Lorentz force distributions, weld pool dimensions and seam profiles. The simulation data agrees well with the experimental results.

Macro-micro modeling and simulation on columnar grains growth in the laser welding pool of aluminum alloy

The heat transfer and columnar grains growth in the laser welding pool of 2A14 aluminum alloy under different welding parameters are investigated through macro-micro modeling and simulation. A macroscopic heat transfer model is proposed to calculate the temperature field in the laser welding pool under different welding parameters. Computed fusion profiles agree well with experimental measurements. The equations of transient pulling velocity, temperature gradient and columnar grains growth orientation during solidification of the laser welding pool are developed. A phase field model coupled with transient pulling velocity, temperature gradient, columnar grains growth orientation and the flow field is developed to simulate columnar grains growth in the laser welding pool under different welding parameters. Laser welding experiments are also carried out to observe columnar grains growth in the laser welding pool under different welding parameters. Simulated columnar grains morphology, spacing and growth orientation give a good agreement with experimental measurements. Competitive growth of columnar grains with different growth orientations is also predicted by the phase field model. Predicted competitive growth process is in accordance with experimental findings.

Dissimilar steels laser welding: Experimental and numerical assessment of weld mixing

Upcoming strict CO2 regulations lead car manufacturers to look for mass saving solutions. The use of advanced high strength steel (AHSS) solutions enable optimizing both crash performances and mass saving. Particularly, the use of laser welded blanks made of dissimilar high strength steels is an efficient weight optimization solution. To support the joining of AHSS in car body design, a 3D model of heat transfer, turbulent flow and transport of species in the laser weld pool has been developed. It aims at providing a better understanding of diffusive-convective mixing in the weld and its influence on the weld mechanical properties. The presented model allows predicting the weld geometry and the element distribution. To validate the model, experimental tests were carried out. Welding of two dissimilar steels with different laser beam offset from the joint line was performed. Numerical and experimental investigations of dissimilar butt laser welding between high Mn and dual phase steels were carried out. The cross sections of the welds were characterized by scanning electron microscope (SEM) with energy-dispersive X-ray spectroscopy (EDX) elemental analysis. Quantitative mappings of Mn distribution in the melted zone offer an overview of mixing intensity. The results of the simulation have been found in good agreement with the experimental data. To go further and to assess the effect of weld mixing on mechanical performances, tensile tests were done. It was found that tensile behavior of the welds is determined by level of Mn and C dilutions. For attaining maximal joint performances, it is necessary to comprehend the elements distribution in the melted zone and to be able to control it through an accurate choice of operational parameters.

Simulation of dendrite growth in the laser welding pool of aluminum alloy 2024 under transient conditions

A macroscopic heat transfer and fluid flow model is developed to calculate the temperature and velocity fields in the laser welding pool of aluminum alloy 2024. The equations of temperature gradient and solidification rate are developed to consider the transient conditions during the laser welding process. The dendrite growth and solute concentration along the fusion boundary are predicted via the phase-field model under the transient conditions. The comparison of the simulation results with the measurements is carried out. Fusion profiles obtained by using a rotary-Gauss body heat source model are consistent with measurements. The computed dendrite morphology and primary dendrite arm spacing give a good agreement with experimental findings.

Multiphysical Modeling of Transport Phenomena During Laser Welding of Dissimilar Steels

The success of new high-strength steels allows attaining equivalent performances with lower thicknesses and significant weight reduction. The welding of new couples of steel grades requires development and control of joining processes. Thanks to high precision and good flexibility, laser welding became one of the most used processes for joining of dissimilar welded blanks. The prediction of the local chemical composition in the weld formed between dissimilar steels in function of the welding parameters is essential because the dilution rate and the distribution of alloying elements in the melted zone determines the final tensile strength of the weld. The goal of the present study is to create and to validate a multiphysical numerical model studying the mixing of dissimilar steels in laser weld pool. A 3D modelling of heat transfer, turbulent flow and transport of species provides a better understanding of diffusion and convective mixing in laser weld pool. The present model allows predicting the weld geometry and element distribution. The model has been developed based on steady keyhole approximation and solved in quasi-stationary form in order to reduce the computation time. Turbulent flow formulation was applied to calculate velocity field. Fick law for diluted species was used to simulate the transport of alloying elements in the weld pool. To validate the model, a number of experiments have been performed: tests using pure 100μm thick Ni foils like tracer and weld between a rich and poor manganese steels. SEM-EDX analysis of chemical composition has been carried out to obtain quantitative mapping of Ni and Mn distributions in the melted zone. The results of simulations have been found in good agreement with experimental data.

Research on Edge Extraction System of the Laser Welding Image Based on ARM

The article researched on edge extraction system of the laser welding image based on ARM micro controller. The system can achieve accurate extraction on the edge of the laser welding pool, and reduce the system cost effectively while satisfying the real-time constraints. With STM32f103 controller for the system as the core, the outside are image storage and display module. The image processing methods of gray-scale transformation, image smoothing, threshold segmentation and edge extraction are realized in ARM, and the processed images are displayed in LCD. The experimental results show that the system researched in this paper can extract color molten pool image edge accurately without distortion and better reflect the relationship between welding parameters and weld pool geometry size, and provide the basis for building real-time control system of the laser welding quality.

Simulation of nitrogen transport and desorption in laser welds of 21Cr–6Ni–9Mn stainless steel

Loss of nitrogen is a concern when welding nitrogen strengthened stainless steel alloys. Building on the current understanding of the underlying mechanisms, a three-dimensional simulation of conduction mode laser weld pool development using the volume of fluid technique was developed. Weld pools formed by a moving Gaussian heat input for two different laser power densities were simulated and the transport and surface desorption of nitrogen was tracked using nitrogen macroparticles. The penetration depth and width of the weld pool predicted by the simulation was comparable to the data derived from macrographs of welds made on nitronic 40 alloy. Additionally, the 25–32% predicted decrease in nitrogen composition of the weld fusion zone by the new rate law is comparable to the literature.

Measurement of laser welding pool geometry using a closed convex active contour model

The purpose of this study was to develop a computer vision method to measure geometric parameters of the weld pool in a deep penetration CO2 laser welding system. Accurate measurement was achieved by removing a huge amount of interference caused by spatter, arc light and plasma to extract the true weld pool contour. This paper introduces a closed convex active contour (CCAC) model derived from the active contour model (snake model), which is a more robust high-level vision method than the traditional low-level vision methods. We made an improvement by integrating an active contour with the information that the weld pool contour is almost a closed convex curve. An effective thresholding method and an improved greedy algorithm are also given to complement the CCAC model. These influences can be effectively removed by using the CCAC model to acquire and measure the weld pool contour accurately and relatively fast.

Study of 3D Reconstruction of Laser Welding Pool Surface Based on Shape from Shading

Shape from shading (SFS) is one of the critical techniques in 3D shape recovery in computer vision. The task of SFS is to reconstruct 3D shape of the visible surface of an object from one single picture using its gray variation. Analyzing the 3D shape of welding pool is important to evaluate the welding quality. During the disk laser welding experiments, a high-power laser was used as the auxiliary illuminant, and a high-speed image acquisition system with infrared filter was applied to capture the welding pools in real time. The slant and tilt of illuminant source were estimated by statistics to reconstruct the 3D shape of welding pool visible surface by using the localization method of SFS. Methods of median filter and cubic spline interpolation were used to denoise images and smooth the image shapes. Experimental results showed that the proposed technique could reconstruct parts of welding pool effectively.

Real-time monitoring of laser welding of galvanized high strength steel in lap joint configuration

Two different cases regarding the zinc coating at the lap joint faying surface are selected for studying the influence of zinc vapor on the keyhole dynamics of the weld pool and the final welding quality. One case has the zinc coating fully removed at the faying surface; while the other case retains the zinc coating on the faying surface. It is found that removal of the zinc coating at the faying surface produces a significantly better weld quality as exemplified by a lack of spatters whereas intense spatters are present when the zinc coating is present at the faying surface. Spectroscopy is used to detect the optical spectra emitted from a laser generated plasma plume during the laser welding of galvanized high strength DP980 steel in a lap-joint configuration. A correlation between the electron temperature and defects within the weld bead is identified by using the Boltzmann plot method. The laser weld pool keyhole dynamic behavior affected by a high-pressure zinc vapor generated at the faying surface of galvanized steel lap-joint is monitored in real-time by a high speed charge-coupled device (CCD) camera assisted with a green laser as an illumination source.

Laser weld pool management through diffractive holographic optics

Conduction laser welding involves initiating a melt pool by exposure to high power laser induced light and controlled thermal conduction. Existing welding techniques generally provide enough energy to join the component but have no real control over the melt pool. This process can invariably lead to overheating in adjacent areas or even the melt pool itself, often causing unavoidable effects, such as ‘burn through’. The present work presents a procedure in which a desired melt pool shape is conceived, and a bespoke beam irradiance distribution is designed to match. The beam is shaped not by conventional lenses but by a diffractive holographic optical element (DHOE). The DHOE utilises holography to wholly create highly complex three-dimensional energy distributions through constructive and destructive interference. This technique allows novel beam irradiance distributions to be applied to conduction mode laser welding, with the melt pool transverse profile being shaped to a specific design. Holographic conduction laser welding has been shown to be successful and represents a significant step forward in the industry, as demonstrated in this case in both mild and stainless steels. The fusion zone is shown to be particularly influenced by the shape of the illuminating laser beam profile, and many of the welds demonstrate a highly novel weld profile because of this. The use of a bespoke beam irradiance distribution allows control of the heat flow to the workpiece, and this allows greater control over material migration due to surface tension effects. Many of the welds demonstrate unique surface solidification patterns directly linked to the beam profile used. The DHOE also presents a number of additional advantages, such as an increased usable depth of field, allowing for less stringent set-up tolerances. Comprehensive metallography has been performed on samples of these welds through the use of optical microscopy, electron microscopy, electron backscatter diffraction and energy dispersive (X-ray) spectroscopy. These techniques offer in depth analysis of crystal size, shape, orientation and phase. By incorporating DHOEs into a laser welding process, not only does the melt pool shape become controllable, but also the crystal growth is highly influenced. Many of the undesirable attributes of a conventional laser weld are reduced by using a beam distribution created by a DHOE, bringing the microstructure of the weld pool closer to that of the parent material.

Heat Source Model of Lap Laser Welding of Stainless Steel Vehicle

Resistance spot welding (RSW) is being taken place by partial lap laser welding for the poor surface quality and bad airtight due to the pressure of electrodes. The shape of partial lap laser welding is similar to the vase. When the penetration of the joint is in a certain range, there is no welding trace on the outer surface. Laser welding temperature field numerical analysis based on Abaqus finite element analysis software is committed to obtain a suitable range of process parameters to improve production efficiency and automation by determining the joint penetration. To master the laser lap welding of stainless steel weld penetration state, the combination of three-dimensional positive cone + three-dimensional inverted cone + half-ellipsoid heat source model was established simulating stainless steel lap laser weld pool shape and forecasting the range of process parameters .

Weld pool flows during initial stages of keyhole formation in laser welding

Weld pool transport phenomena during the transition from conduction-mode laser spot welding to keyhole laser spot welding of titanium were studied by numerical simulation. A range of laser powers were simulated and temperature dependent evaporation recoil pressure and cooling were applied as boundary conditions on the weld pool surface. Simulation results predicted a complex time-varying flow pattern during weld pool development. The surface-normal flow at the weld pool centre oscillated between upwards and downwards during the simulation time due to interaction of competing effects of evaporation recoil and surface tension pressures and laser heating and evaporation cooling. The results show that the laser weld pool flow dynamics play a key role during the transition from conduction-mode laser welding to keyhole welding.

Use of a multivariate optimization algorithm to develop a self-consistent numerical heat transfer model for laser spot welding

Laser spot welding as a joining method offers many outstanding advantages, such as localized heating and melting, high weld-strength-to-weld-size ratio, and minimal heat affected zone. These provide the benefits of low heat distortion, repeatability, ability to automate and high throughout that are always in demand in industry. An accurate knowledge of the temperature-time history of the weld pool is a prerequisite for reliable prediction of the weld dimensions, final microstructure and mechanical properties of the weld joint. Measurement of the weld thermal cycle in the laser weld pool is nearly impossible due to high peak temperature, rapid melting and solidification, and the complex flow of liquid metal within a small weld pool. Mathematical modeling of the laser spot welding process has emerged as a useful tool for the prediction of the temperature-time history and weld pool dimensions. However, the reliability of the predicted values of temperature history and weld dimensions significantly depends on the accuracy of the input parameters provided in such models. For example, the value of the absorption coefficient is a significant input parameter for modeling the laser spot welding process. However, the same is rarely available with adequate reliability and is also difficult to assign from scientific principles alone. This work presents a novel mathematical framework where the values of a set of uncertain input parameters for mathematical modeling are identified inherently by integrating a finite element based heat transfer simulation using adaptive volumetric heat source and a multivariate optimization algorithm.

Effect of pressure gradient driven convection in the molten pool during the deep penetration laser welding

The pressure distribution around the keyhole during the deep penetration laser welding is simulated. The simulation physically corresponds to the process of a focused laser beam irradiating on the surface of a moving workpiece. A calculation of the velocity vectors and absolute pressure near the front wall and dynamic pressure in the laser weld pool shows that the pressure gradients is directly related to the velocity magnitude of the pool fluid flow, and the dynamic pressure gradient against the welding direction at the rear of weld pool is a retarding force. The velocity of fluid flow near the real wall of the keyhole is smaller, and the formation of the shoulder in the weld pool is contributed by a combination of the movement of the keyhole relative to the workpiece which forms a force that is called expellant force and the friction force of the metal vapor escaping from the keyhole and buoyancy.

Mathematical formulation and simulation of specular reflection basedmeasurement system for gas tungsten arc weld pool surface

Weld pool surface can change dynamically during welding and is indicative ofinformation critical to controlling the process. Research has picked up in the fieldof observing the weld pool surface to understand the dynamics of thewelding process. This paper will help visualize and understand the physicsinvolved in observing the weld pool surface. A study of laser properties,weld pool and camera optics was incorporated in developing a model todescribe the mechanism of observing the weld pool surface from specularreflection. This observation method projects a laser beam on the pool surfacethrough an optical grid with a frosted glass attached. The correspondingspecular reflection is calculated, which is derived based on the reflection law.The reflected laser beams are then captured by the camera to form theimage. The model can be used to predict the outcome of experiments withgrids placed in front of the laser and to determine the position where thecamera should be placed to acquire the best image. Preliminary resultsshowed that the camera should be placed with the weld pool along theoptical axis, and the aperture should be as large as possible to allow asmany rays into the camera as possible. The model can be used to findthe optimal location of the laser and camera for materials of differentthickness, by moving the electrode higher in the simulation, and adjusting thelaser and camera location accordingly. The paper will give some insightinto problems that might be encountered in observing the weld pool, andsuggest the set-up of the laser and camera for obtaining the best image.

Phase change problem and effects of geometrical constants on weld pool geometry in sheet metal laser welding

A three-dimensional quasi-state heat conduction model is presented for the solid/liquid interface in laser welding of thin sheet metals. The vapour plasma plume effect is incorporated into the model by considering the electromagnetic field equation to determine the laser power reaching the substrate surface after passing through the plume region. Optical properties, such as the dielectric constant and electrical conductivity of plume, and the thermal conductivity of the vapour of stainless steel 316 in the plume region are estimated. Elliptic-paraboloid weld pool geometry is considered and analytic expressions for temperature distributions are obtained in the solidand liquidregions based on theweld pool geometry. Experimental and theoretical results for the weld depths and widths areillustratedfor process parameters such as the laser power, welding speed, and weld pool shape factors in x and y directions.